EARTH SURFACE PROCESSES AND LANDFORMS Earth Surf. Process. Landforms 39, 2100–2108 (2014) Copyright © 2014 John Wiley & Sons, Ltd. Published online 2 July 2014 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/esp.3607 Different morphology of Nuphar lutea in two contrasting aquatic environments and its effect on ecosystem engineering Jonas Schoelynck,1* Kris Bal,1,2 Veerle Verschoren,1 Ellis Penning,3,4 Eric Struyf,1 Tjeerd Bouma,5 Dieter Meire,6 Patrick Meire1 and Stijn Temmerman1 1 Department of Biology, Ecosystem Management Research Group, University of Antwerp, Wilrijk, Belgium 2 Department of Biodiversity, School of Molecular and Life Sciences, University of Limpopo, Sovenga, Limpopo, South Africa 3 Deltares, Delft, The Netherlands 4 Netherlands Institute of Ecology, Wageningen, The Netherlands 5 Netherlands Institute of Ecology, Centre for Estuarine and Marine Ecology, Yerseke, The Netherlands 6 Department of Civil Engineering, Labo voor Hydraulica, University of Ghent, Ghent, Belgium Received 17 April 2013; Revised 28 May 2014; Accepted 28 May 2014 *Correspondence to: Jonas Schoelynck, Department of Biology, Ecosystem Management Research Group, University of Antwerp, Universiteitsplein 1C, B-2610 Wilrijk, Belgium. E-mail: [email protected] ABSTRACT: Aquatic plants (macrophytes) can have a large effect on river hydraulics and geomorphology. Though, the extent to how plant morphological plasticity actively influences these feedbacks has received little scientific attention. The nymphaeid macrophyte species Nuphar lutea (L.) Smith is characterized by a distinct leaf duality. Floating leaves shade most of the submerged leaves thereby limiting light penetration in the water. Despite their apparent negligible photosynthetic role, submerged leaves of N. lutea remain intact during summer and contribute a significant part to the total biomass. Our results indicate that the submerged leaves are crucial in plant–flow interactions and hence in the engineering potential of the plant, i.e. the capacity to locally reduce flow velocities and to promote sedimentation, including organic matter deposition. Plant individuals growing in running river water were compared to individuals from adjacent oxbow lake water. The number and size of submerged leaves were significantly higher for river standing individuals and the accumulated sediment contained significantly more organic matter, total nitrogen and total phosphorus, and was characterized by a lower carbon/nitrogen ratio and a finer grain size. We therefore argue that the submerged N. lutea canopy in rivers has the ability to create a high-nutrient, low hydrodynamic environment, resembling the conditions found in oxbow lakes. Copyright © 2014 John Wiley & Sons, Ltd. KEYWORDS: Biebrza; scale-dependent feedbacks; morphological plasticity; bio-geomorphology; aquatic vegetation; nutrient dynamics; organic matter accumulation Introduction deviation around the patches (Schoelynck et al., 2012). As a consequence, shoots inside patches have a better survival Multiple studies have demonstrated that plants can significantly chance and a larger primary production than shoots emerging affect river channel form and adjustment (Tal and Paola, 2007; between patches, where erosion, scouring and uprooting can Hicks et al., 2008; Larsen and Harvey, 2010, 2011; Montakhab limit here plant growth and patch expansion (Schoelynck et al., 2012). There is a growing scientific interest in these et al., 2012). Macrophyte patches thus create spatial variability plant–river interactions (e.g. Franklin et al., 2008; Gurnell in stream velocity and induce geomorphological changes of the et al., 2010; Zong and Nepf, 2010; Nepf, 2012), focusing river including (i) changes in bathymetry (Cotton et al., 2006; mostly on riparian plant species, particularly shrubs and trees. Schoelynck et al., 2012; Schoelynck et al., 2013) and (ii) bank However, true aquatic macrophytes can also affect the hydrau- erosion leading to meandering (Gurnell, 2014, and references lics and geomorphology of rivers (Gurnell et al., 2010; Bertoldi cited therein). et al., 2013). Growing in patches, they can act as ecological Apart from river studies, knowledge of vegetation– engineers (Gurnell, 2014) meaning that they are capable of hydrodynamics–geomorphology interaction is also rapidly adapting the environment to their benefit, with positive and/or emerging from studies in coastal and estuarine environments. negative feedbacks. Flow velocity, for instance is reduced Bouma et al. (2007); Temmerman et al. (2007); Van Wesenbeeck inside macrophyte patches, as friction is generated by the can- et al. (2008) and Vandenbruwaene et al. (2011), for instance, opy (Vandenbruwaene et al., 2011; Schoelynck et al., 2012). clearly showed differences in stream velocity in and around Adjacent to patches, flow velocity is increased because of flow vegetation patches of the tidal marsh grass Spartina anglica C. MORPHOLOGICAL PLASTICITY AIDING ECOSYSTEM ENGINEERING 2101 E.Hubb, causing increased sedimentation in the patches, and (McMillen and McClendon, 1979; Binzer and Sand-Jensen, eroding gullies around the vegetation. Bouma et al. (2009) 2002). However, floating leaves covering most of the sub- proposed a size-dependence of these plant–flow interactions, merged leaves (Figure 2) are likely to limit light penetration in with clear threshold values as they state that a minimum patch the water and the photosynthetic potential of the submerged size and minimum organism density within the patch is needed leaves. Still, despite this hypothesized negligible photosyn- before habitat modification can occur. This means decreasing thetic role, submerged leaves of N. lutea remain intact during flow velocities within the vegetation (i.e. positive feedback) summer and contribute a significant part of the total biomass and increasing flow acceleration alongside the vegetation patch (Snir et al., 2006). Flow velocities may evoke strong hydrody- (i.e. negative feedback). The combination of positive feedbacks namic forces on the (submerged) shoots and leaves, obliging within and negative feedbacks outside the vegetation are gener- the plant to invest in energy-expensive tissue reinforcement to ally referred to as scale-dependent feedbacks (Rietkerk and Van withstand them (Schoelynck et al., 2010): the presence of sub- de Koppel, 2008). Such feedbacks were recently also demon- merged leaves thus seems counterproductive. Other rooted, strated to result in spatial self-organization of riverine vegetation floating-leaved species like Potamogeton natans L. minimize (Schoelynck et al., 2012). their submerged biomass and concentrate all leaves at the wa- The extent to which plant morphological plasticity can ac- ter surface which maximizes their photosynthetic success and tively influence these scale-dependent feedbacks has received minimizes hydrodynamic forces (Bal et al., 2011). little scientific attention. The nymphaeid macrophyte species We argue that the submerged leaves are crucial in plant–flow Nuphar lutea (L.) Smith (Figure 1) is characterized by a distinct interactions (Bal et al., 2011; Puijalon et al., 2011) and hence in leaf duality. Nuphar lutea is common in the temperate regions the engineering potential of the plant, i.e. the capacity to alter of the northern hemisphere (Heslop-Harrison, 1955): it occurs flow velocities and to promote sedimentation including organic both in lakes with little hydrodynamic stress as well as in matter deposition. Higher sediment organic matter content may flowing streams. They prefer depths between 0.6 m and 2.4 m lead to higher nutrient and carbon availability (Brock et al., (Heslop-Harrison, 1955). Translucent, crumpled submerged 1985; Webster and Benfield, 1986; Snir et al., 2006), improving leaves are produced in early spring; leathery floating leaves plant productivity within the patches. This is especially impor- emerge later and reach the water surface in April and May. tant for an eutraphent species like Nuphar lutea (Bornette and The occurrence of submerged leaves during the whole growing Puijalon, 2011). season is less explicitly found among other nymphaeid species By means of field measurements, we investigate two possible (Smits et al., 1988), and the reason for this summer presence is reasons for the presence of submerged leaves on Nuphar lutea unstudied. Shoot density and leaf orientation in vegetation are individuals at periods of peak biomass. We hypothesize that: classically explained in relation to photosynthetic success Hypothesis 1 Nuphar lutea is an ecosystem engineer: the submerged leaves are efficient in reducing flow velocity thereby increasing organic matter deposition and hence nutrient availability. The efficiency of reducing the flow velocity depends on size and density of the patch: investing in submerged leaves is hence only valuable in running river water and not in still oxbow lake water, implying morphological Figure 2. Picture of a Nuphar lutea patch in the river Sidra, near the village of Harasimowicze, which is location 1 in Figure 3. This patch is representative of all patches measured in the river in this study, though blockage factor (ratio patch width to river width) may vary Figure 1. Nuphar lutea (L.) Smith individual from the Biebrza Na- among the different locations. Floating leaves cover most of the water tional Park (Poland). Picture was taken outside the water lying on a surface and shade the submerged leaves to a great extent. To the right white background. Starting